Recently, a laminated metal sheet, in which synthetic resin films such as polyester films are laminated on one side or both sides of metal sheet such as thin steel sheet or plated thin steel sheet, is applied for manufacture of sheet materials for the production of beverage cans and others. In case of forming of this laminated metal sheet into a can by deep-drawing forming, the laminated resin film protects the metal sheet from corrosion caused by the contents in the can. Further, when the laminated metal sheet is formed, there is an advantage that it does not need to be coated with lubricant as is required for uncoated metal sheet and to be, subsequently rinsed free of the lubricant, since the synthetic resin film functions as a lubricating layer.
When the laminated metal sheet is formed by deep-drawing, cracks and pinholes are sometimes generated in the synthetic resin film depending on the forming conditions. As these cracks are formed right under the flange portion, the appearance becomes unattractive. Further, the metal substrate is apt to corrode where the cracks occur, which causes durability trouble. Accordingly, various suggestions on preventing cracks have been provided, such as partial controlling of the orientation degrees of the synthetic resin film, increasing of adhesion between the metal substrate and the synthetic resin film and others (refer to International Application No. PCT/JP94/01260 and others). Generation of cracks could be substantially reduced by such means.
However, pinholes generated at random could not sufficiently be reduced even by the above-mentioned means, and the metal substrate could not sufficiently be prevented from being corroded by the contents oozing through pinholes in the plastic coating of the can, which deteriorates durability. It is the objective of the present invention to provide a laminated metal sheet in which generation of pinholes can be minimized in the deep-drawing forming and others, and the durability of cans for bevarages and others to be improved thereby.
It was found that the incidence of the above-mentioned pinholes can be reduced to some degree by changing the condition of the synthetic resin film and the condition of the surface roughness of the metal sheet. However, some elements remained almost unaffected by the change in condition of the base materials. Thus, we noticed the existence of fine particles floating in the air, as the condition other than those related with the base material. Namely, we considered that since such fine particles are adsorbed on a film surface and thereby exist between a metal sheet and a film, pinholes are generated originating from fine particles when the laminated metal sheet is formed into a can. And as a result of the cleanliness of air being heightened in the environment of the laminating (doubling) process, it was recognized that there is a high correlative relation between the class cleanliness (Federal Standards No. 209D), the standard of cleanliness of air, which shows the number of fine particles beyond 0.5 .mu.m existing within a space of 1 ft .sup.3, and the incidence of pinholes. Further, we considered that fine particles having a mean particle diameter below 3 .mu.m do not mainly attribute to generating pinholes, and we noticed that for controlling the generation of pinholes, it is more desirable that standards for measuring the cleanliness in the environment of the laminating (doubling) process is based not on the Federal Standards, but on the concentration of fine particles having a mean particle diameter exceeding 3 .mu.m, and also that an excess of the cleanliness of air is not needed. These matters led us to accomplish the present invention.